The invention relates to electronic detection derived from correlating changes in dielectric field strength. More particularly, it applies to systems that detect the presence of certain classes and combinations of material as the material accumulates on a surface, e.g., ice buildup on the surfaces of aircraft.
Ice accretion on the wings of fixed-wing and on the rotors of rotary-wing aircraft can have disastrous results. The ice that forms on a wing structure, especially along the leading edge, modifies the aerodynamics of the wing, resulting in decreased lift. In the extreme, this can lead to loss of lift and control of the aircraft and potentially a crash. Ice accumulating elsewhere on the wing and airframe can add significant weight to the aircraft. Several techniques and flight protocols have been developed and are widely used to prevent a plane from becoming ice covered, both in flight and on the ground.
Some, typically larger aircraft, are equipped with in-flight heaters that melt the ice before it can substantially build up on wings or rotor blades. Protocols have been established for permitting or denying flight under weather conditions or into areas where the potential of aircraft icing is high. On the ground, there are deicing protocols and methods that ensure that there is little to no accretion of ice on wings or rotors immediately prior to flight.
An outstanding problem is that it is difficult while in flight or on the ground to determine when ice is building up on the aircraft until a substantial accretion has taken place. By that time, it may be difficult or even too late to take evasive maneuvers or rely on the in-flight deicing capability.
On the ground, it would be useful to monitor the state of wing and airframe coverage by deicing fluid, liquid water or the accretion of ice. Availability of this information can be used to decide when to implement deicing procedures with greater efficiency and economy.
Current icing detectors using radio frequencies (RF) in transmission lines are single point detectors. For example, U.S. Pat. No. 5,695,155, Resonator Based Surface Condition Sensor, issued to; MacDonald et al., Dec. 9, 1997, uses multiple microstrip resonators, one for each point, positioned to couple with an RF energized transmission line. The resonators produce amplitude minima in the RF signal, the resonance changing dependent upon the makeup of the dielectric covering the microstrips. By fabricating each microstrip to have a different resonant frequency and knowing where each is installed, the location of material accretion can be identified.
U.S. Pat. No. 5,772,153, Aircraft Icing Sensors, issued to Abaunza et al., Jun. 30, 1998, employs complex phase detection circuitry with a parallel electrode xe2x80x9csurface gap transmission linexe2x80x9d that must be affixed to various locations of interest on a surface, one for each point. In a preferred embodiment the surface gap transmission line is energized with an electric field of varying frequency that is reflected by a ground (conducting) plane upwards into a volume immediately above and between the two electrodes. It determines the makeup of the material on the surface, if any, by detecting phase changes in the RF signal passing down the two electrodes and reflecting upwards from the ground plane and converting these phase changes to xe2x80x9cpropagation timesxe2x80x9d to correlate to changes in the square root of the dielectric constant of the media through which the reflected RF signal passes. Temperature data may also be used to provide an unambiguous determination of the material. One embodiment also uses an identical second sensor system as a reference, eliminating the need to determine temperature.
Other ice detectors use acoustics, heat, light, or a combination thereof, e.g., U.S. Pat. No. 5,467,944, Detector For Indicating Ice Formation on the Wing of an Aircraft, issued to Luukkala, Nov. 21, 1995 is based on a thread-like or a tape-like transducer, through which an ultrasonic signal is transmitted at one end. The attenuation of a signal passing through the thread is measured with a receiver at the opposite end while the thread is simultaneously being heated such that ice that may surround it melts again, the attenuation thus resuming its initial level. U.S. Pat. No. 5,629,485, Contaminant Detection System, issued to Rose et al., May 13, 1997, transmits ultrasonic signals through a surface xe2x80x9cskinxe2x80x9d and collects data based on propagation of these signals through the skin and the dispersion curves representing natural resonance of the signals on an xe2x80x9cunloadedxe2x80x9d surface skin. Knowing a priori the response of an xe2x80x9cunloadedxe2x80x9d skin and a catalog of responses for one that is loaded with a variety of materials, e.g., water, ice, glycol, and combinations thereof, a detector and warning system may be applied to various applications, e.g., buildup of ice on an airframe.
A preferred embodiment of the present invention provides a continuous indication of the presence or absence of a buildup of material on a surface, e.g., liquid water, glycols or ice alone, or mixed phase liquid water, glycols and ice, over large areas of an airframe. Additionally, it has the potential to indicate the presence of at least a pre-specified minimum level of a contaminant on any region of a surface instrumented with a preferred embodiment of the present invention.
A system is provided for detecting accumulation of types of material, including combinations of types, upon multiple areas of a surface concurrently. In one preferred embodiment, it uses a single long wire conductor having a pre-specified characteristic impedance. At one end of the conductor an energizing source is connected while at the opposite or distal end the conductor is configured to have a xe2x80x9ctermination impedancexe2x80x9d different from the conductor""s characteristic impedance. For this embodiment, an electromagnetically conducting ground plane is employed. The ground plane abuts the conductor but is electromagnetically isolated from it. The ground plane may be part of the surface being instrumented if that surface is a good conductor. A second preferred embodiment does not require an adjoining ground plane, but uses another similar conductor run parallel and in the same plane as the single conductor or the single wire configuration. This is useful when the surface comprises a strong dielectric such as fiberglass.
A major part of the system is the sub-system comprising a reflectometer, either a Time Domain Reflectometer, including commercial models, or an FM-CW reflectometer. One function of the reflectometer is to provide the analog signal that energizes the conductor, typically a transmission line. It also processes the reflection of the analog signal from the distal end as well as partial reflections from any dielectric discontinuities present at boundaries indicative of accumulation of material on the surface above the conductor. The processed reflected signal, combined with a portion of the original signal, yields information for decision making.
A number of configurations can be used for the transmission lines, e.g., conducting tape electromagnetically insulated on one side, striplines, electromagnetically insulated wires, coaxial cable, and substrates having at least a dielectric layer and an electromagnetically conducting layer.
The TDR may be fabricated from components. One example uses a generator for providing a pulse of narrow pulsewidth and appropriate repetition frequency; a circulator for coupling the pulsed signals to the conductor and coupling the reflections from the conductor to the TDR; and a processor for processing the signals and displaying results, such as an oscilloscope.
The FM-CW reflectometer may be constructed from the following components: a linear sweep generator for generating the FM-CW analog signal; a circulator for coupling the FM-CW signal to the conductor and the reflected signals from the conductor to the reflectometer; a mixer for combining the reflected signal with a portion of the initial analog signal; a low pass filter for passing only the low frequency spectra, typically audio, of the mixed signal; a high pass audio filter for passing only the highest frequency spectra of the audio signal; an audio amplifier for amplifying the high frequency audio signal prior to digitizing it; an analog-to-digital converter (ADC) for converting the analog audio signal to digital format; a digital signal processor (DSP) for further processing to extract data on features of the reflected analog signal and a computer for comparing data on features of the reflected analog signal with reference data and displaying results.
Use of either of these embodiments enables alerting to accumulation of material on an instrumented surface as it occurs and concurrently for all instrumented surfaces. In a preferred embodiment of the present invention, a single reflectometer may operate multiple transmission line sensors by multiplexing the analog reflection signals from each transmission line sensor.
The method for detecting accumulation of material on multiple areas of a surface concurrently includes:
emplacing a transmission line on the surface such that it is electromagnetically isolated from the surface;
providing an electromagnetically conducting ground plane adjacent to the transmission line or another similar transmission line placed a pre-specified distance from the first and parallel thereto, both of which are electromagnetically insulated from each other;
energizing the transmission line(s) with an appropriate analog electromagnetic signal, either a pulsed signal or FM-CW, at one end of the transmission line(s);
receiving at the same end as the source of the energizing at least one reflection of the analog electromagnetic signal from the far, or distal, end of the transmission line(s); and
extracting at least one quantifiable feature from the reflected signal.
In one embodiment, a value associated with the extracted quantifiable feature may be compared to a reference value to detect a particular material, e.g., ice, or material type, e.g., a glycol or glycol solution, that may be present in a pre-specified amount, i.e., thickness, on the surface immediately above the transmission line sensor.
The pulsed signal used with the TDR may be provided at a peak signal level of between 0.05 V and 10 V, a carrier frequency of between 1 MHz and 40 GHz, a pulse width of between 0.1 nanosec and 10 millisec, and at a pulse repetition frequency (PRF) of between 0.01 Hz and 1 MHz. Typical commercial models operate a 900 MHz half-cosine pulse shape with pulse widths of 2 nanosec at a 5V peak.
Stepped versions may operate within the same carrier frequency range as pulsed versions, but with rise times from 100 picosec to 100 nanosec and an amplitude step from 10 mV to 10 V. A commercial stepped version has a typical rise time of 200 picosec and an amplitude step of 300 mV at a carrier frequency of 900 MHz.
The FM-CW signal may be provided at a signal level of between 1 mW and 10 W, at a carrier frequency of between 1 MHz and 40 GHz, a bandwidth of between 10 and 60% of carrier center frequency and is swept linearly in carrier frequency at a pre-specified period of between 100 millisec to 10 millisec. A preferable embodiment operates at a signal level of 10-100 mW, a carrier center frequency of between 100 MHz and 1 GHz, with a bandwidth of from 30-50% of the carrier center frequency, and is swept linearly at a period up to 10 millisec.
In one embodiment, the processing by the reflectometer provides at least a measure of the round trip time interval for the analog signal to travel from the source end to the distal end of the transmission line and return as at least a partial reflection from the distal end. It may also provide at least a measure of the time for the analog signal to travel from said source end to a first location along the transmission line comprising a dielectric boundary that generates a partial reflection of the signal before it gets to the distal end. This location is defined by the closest boundary to the source end of a first region around the transmission line that incorporates a material other than the medium surrounding the transmission line in a reference state.
Additional information may be obtained by employing a spectrum analysis algorithm to extract said at least one feature, e.g., the type of material, the thickness of the accumulation, or the location(s) of the accumulation. Further, the identification of material type and thickness of accumulation may be facilitated by the provision of a reference, such as a look-up table, associated with the processing electronics. The combined use of algorithms and references may provide information on: thickness of accumulation upon the surface, location of accumulation upon the surface, type of material accumulating upon the surface, mixes of material types accumulating upon the surface, rate of accumulation of material upon the surface, and combinations thereof.
Further, a preferred embodiment of the present invention provides for conveying the information to a decision maker, either an automated alerting system or directly to a human operator, such as an aircrew or ground crew member. Finally, a preferred embodiment of the present invention may permit prediction of the occurrence of an accumulation, given the use of suitable algorithms to manipulate information able to be provided by the transmission line sensors.
Implementation of this system solves the following problems for private, industrial, commercial and municipal aviation users:
detection of liquid water on aircraft wing or rotor;
detection of ice accretion on aircraft wing or rotor;
detection of dry aircraft wing or rotor;
detection of the presence of deicing agent on aircraft wing or rotor;
wet/dry condition of wings and rotors on ground and in flight;
condition of spray-on deicing agent;
integration of time domain reflectometry with structure of aircraft wing or rotor; and
integration of FM-CW reflectometry with structure of aircraft wing or rotor.
Advantages of preferred embodiments of the present invention, as compared to conventional systems, include:
concurrent remote electronic indication and measurement of presence of liquid water, ice or deicing agent on multiple aircraft surfaces either in flight or on the ground;
in-air sensing of icing conditions of aircraft surfaces, including wings and rotors;
on-ground sensing of icing conditions of aircraft surfaces, including wings and rotors;
economical installation with low lifecycle cost;
ease of installation and use;
installation without substantial restructuring of surfaces and the vehicle;
resistant to mechanical and environmental stresses of aircraft operation;
provides a quantified estimate of the time required between de-icing events;
increased flexibility for use;
may be used to predict icing events;
allows de-icing materials to be applied in the location and amounts
needed, when needed without wholesale application of hazardous materials;
high reliability and low false alarm rate;
alternate configurations available; and
ready upgradability to state-of-the-art improvements.
Embodiments of the present invention may be applied to any operation where knowledge of actual or potential material accumulation on a surface is valuable. Use of this apparatus may be applied in manufacturing processes, to detect buildup of unwanted material such as ice or snow on roads, runways, power lines, load-bearing members, roofs, bridges, etc. Numerous industrial, commercial, municipal, and military aviation applications may take advantage of this concept, alone or in concert with other mechanisms such as alarms or heaters, for example.
Preferred embodiments are fully disclosed below, albeit without placing limitations thereon.